The present invention relates to a hydraulic control system for hydraulically-operated excavators, shovels, or other similar heavy-duty earth-working machines. These machines use a plurality of fluid pumps, at least one of which being associated to the turning device of the earthworking machine, turning it around a vertical axis, while the others are associated with other power equipment such as the drive-wheels, power tools, and so forth, all being operated by hydraulic cylinder pistondrives in more or less similar arrangements.
Hydraulic control systems of the type to which the invention pertains, for example, are shown at least in parts in German Printed Patent Application No. 16,34,790. The control system as therein described includes a switch valve associated with an arrangement in the discharge line of the turning equipment so as to avoid that the pump operating the turning facilities has to work against the pump of the other hydraulic cylinder and piston arrangement therein. The switching valve will change position upon the occurence of a particular pressure differential. That differential pressure may have different causes. If the switching valve is not operated, the motors on the feed as well as on the discharge sides are simultaneously operated at a high pressure, and that, of course, is impractical and disadvantageous from the point of view of the use-life of these motors.
In the case of hydraulic excavators with a three-circuit hydraulic system, one usually employs two pumps for the power tools, and a separate circuit for turning the excavator. The turning equipment is associated with a pump and will just idle along when not turning the equipment. Since separate and different control characteristics are necessary for the different tasks and purposes and therefore circuits, this three-circuit hydraulic arrangement must reflect these differences in control tasks and operating characteristics.